The present invention relates to a slip-controlled brake system provided for automotive vehicles with driven front axle and rear axle and which is equipped with a pedal-actuated braking pressure generator comprising a power brake booster connected to an auxiliary pressure source. The booster communicates directly with one or several wheel brakes and which acts upon a master cylinder to the working chambers of which the other wheel brakes are connected. The brake system includes electromagnetically actuatable multi-directional control valves contained in the pressure fluid conduits leading to the wheel brakes and includes pressure fluid return lines which connect the wheel brakes with a pressure supply reservoir and which contain likewise electromagnetically actuatable multi-directional control valves. The system further comprises a normally closed pressure fluid conduit from the power brake booster to the master cylinder which can be switched to its opened position and additionally comprises wheel sensors and electronic circuits for the determination of the wheels' rotational behavior and for the generation of valve control signals.
Slip-controlled brake systems of the type initially referred to are known and which prevent locking of the controlled wheels by virtue of control of the multi-directional control valves which are inserted in the pressure fluid conduit from the braking pressure generator to the controlled wheels and in the pressure fluid return lines from the wheels to the pressure supply reservoir. The risks involved when the wheels become unstable or lock, respectively, in particular the risk of skidding and loss of steerability, thereby will be eliminated or at least reduced considerably. However, such systems do not have any influence on the spinning of wheels due to excessive driving torque.
Moreover, it has also been proposed to design hydraulic brake systems of the like such as to limit traction slip as well. To this end, a connecting valve was employed according to the proposal described in German patent application P No. 33 27 401.0, now U.S. Pat. No. 4,565,411 in which because of the valve the driven wheels will be isolated from the braking pressure generator and connected directly to the auxiliary pressure source as soon as traction slip becomes too high. This allows introduction of hydraulic pressure into the connected wheel brakes even without application of the brake.
According to another solution pressure out of the auxiliary pressure source is supplied directly into the master cylinder for the reduction of traction slip. The pressure propagating by way of a prechamber and by way of check valves in the sleeves of the master cylinder pistons into the working chambers of the master cylinder and from there by way of the inlet valves to the wheel cylinders. The individual multi-directional valves in the pressure fluid conduits to the wheel brakes allow to dose the pressure, if necessary with the assistance of the outlet valves which can establish connection between the wheel brake cylinder and the pressure supply reservoir. The driven wheels must be connected to the master cylinder in a like system.
It is an object of the present invention to improve upon a like brake system in a particularly simple manner and requiring minimal effort so that it can be employed for all-wheel-driven vehicles and which not only prevents locking of the wheels but also limits the traction slip of the wheels to an admissible value.